1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, provided with an improved characteristic impedance of the signal transmission line of a semiconductor integrated circuit, capable of reducing coupling with other transmission lines.
2. Background Art
Conventionally, when providing a signal transmission line with a microstrip structure constituted by a signal line and a ground plate in a semiconductor integrated circuit, two separate wiring layers for the signal line and for the ground plate are used as shown in FIG. 8.
In order to increase the characteristic impedance of this signal transmission line in a signal transmission line of a microstrip structure composed of the integrated signal lines and a ground plate, two methods have been adopted, one method is to make the signal line width narrower, and the other method is to increase the distance d between the signal line and the ground plate.
Furthermore, the other conventional technique is disclosed in Japanese Unexamined Patent Application, No. Hei 5-343564, which determines the characteristic impedance of the transmission line by a mesh-like mesh ground plane, and the spaces corresponding to the pores of the mesh are compensated for by the ground plate.
However, when the above-described first method for making the signal line width narrower is adopted, the problem arises that the maximum input capacitance of a circuit connected to the end of the signal transmission line becomes small.
When the above-described second method of increasing the distance d between the signal line and the ground plane is adopted, the problem is encountered that the thickness or the number of wiring layers increases, because it is necessary to increase the thickness of the interlayer films of the wiring layers or to increase the number of wiring layers.
When the third method disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 5-343564 is adopted, although it is possible to increase the impedance of the signal transmission line, the problem arises that, when assembling into an integrated circuit, since the signal lines cannot be formed under the ground plane, it is not possible to avoid generating coupling between a plurality of signal lines.
When coupling occurs, a deficiency arises that in the characteristic impedance of a certain signal line will experience a dynamic change, because the capacitance between one signal line and its counter line changes when the voltage level of the counter signal line changes.
In addition, the problem also arises in the case of using a mesh ground plane that it is difficult to evaluate the characteristic impedance, since the electro-magnetic field generated through small holes or spaces cannot be accurately understood.